My freshman year design teacher taught me many of the most important lessons of my artistic life. At the time, I despised him, but only now, six years later, am I beginning to be thankful for the way he shaped how I see the world.

My teacher — we'll call him K — was beyond fascinating. He was like our own personal Mad Hatter, in both appearance and voice. He'd speak in riddles, doling out seemingly unfounded assignments, like visiting the Penn Station bathroom at 1 a.m. with the intent to create an "interactive environment" based off of the experience. He applauded projects that utilized tampons, confetti, and raw emotion as props. He'd grade not on technique and talent, but rather on the inspiration and emotion that were communicated through our creative works (which made me insufferably angry for the first half of my freshman year).

But, most importantly, he taught me how to see color and, subsequently, undertone.

Each week, we were required to create a drawing using only colored pencils. We started off by sketching household items like socks and jewelry, and oranges.

Eventually, we were told to create a self-portrait. But of course, the Mad Hatter himself couldn't let us go without a confusing catch. We could use all of our color pencils except black and flesh tones.

At first, I was devastated. How could I draw skin without a flesh-colored pencil?! How could I create depth and shadow without black?! Impossible.

I ran home to my dorm, determined as ever to get a good grade in the class (I was a formerly nerdy, compulsive straight-A student getting a B, so I was determined). I stared into the full-length mirror that hung on the back of my door for what felt like an eternity (well, at least an hour). I searched my face for the patches of blue, green, red, and yellow that he promised I'd uncover. And finally, something clicked.

I saw the shades of blue and green cradling my under-eye. I understood the prominent cast of orange around my mouth, and the pink wash on my cheeks and hairline. I was able to create my skin tone solely from the colors of the rainbow. Perhaps most importantly, I saw the equal parts of pink and gold that existed in my skin, mixing and mingling together to create a neutral undertone. (If you're still not sure what I mean, I suggest trying this activity!)

And with that, my perception of color changed. I now see the world (especially the cosmetic world) differently than I had before.

Undertones are a spectrum, ranging from yellow (warm/gold/olive/peach) to red (cool/pink/blue). For example, I'm neutral (equal parts gold and pink), but I skew a hair more toward warm. It is not a simple 1+1=2 type of science. Sometimes hair color and season can shift the way you perceive your undertone. Even lighting affects how an undertone may be understood.

Undertone has to do with the underlying tone in the skin, not how saturated (light/medium/deep) the skin is. The saturation does (obviously) have an impact on the shades we pick, but you can (for the most part) have any saturation with any undertone.

Now that we know what we're looking for, here are some additional ways to determine your undertone.

Warm/Yellow Undertones:

Those with golden undertones can have a greenish cast visible from the veins on their wrists.

Golden undertones generally view warmer tones as more flattering (like warm red, orange, and yellow), while cooler tones can make more of a statement (like green, blue, and blue-toned red).

Clearly, all of these rules are "general." I still recommend truly looking at your face and searching the colors that naturally exist in your skintone. Do you see mostly yellows, oranges, golds, and olives? If so, you probably have a warm skin tone.

Special Shoutout to Olive Undertones: Olive undertones fall under the warm umbrella, but tend to have a greener (hence olive) undertone, rather than a yellow one. Perfect example: Eva Mendes

And to Peach Undertones: Peach undertones fall into the neutral/warm categories. They're generally golden tones with a bit of added pink. Look to Hilary Duff for an example.

Some additionalimportant things to consider:

If you choose the wrong undertone of foundation for the skin, the skin can appear to turn... interesting colors. If you use a foundation that's too warm or cool on a neutral undertone, it can start to turn orange or pink. If you use a color that's warm on a cool undertone, or cool on a warm undertone, it can start to turn gray. Think about color-correcting. If we place opposing colors on top of each other, they can start to neutralize out.

If you tan over the summer, choose a new, deeper shade with the same undertone. Don't just go warmer or pick the next foundation in the shade range. Many foundations (like Lancome, Urban Decay, and Tarte for example) don't divide their foundations by undertone on their displays, so if you just pick the next one in the series, chances are you're picking one with the wrong undertone.

Determining your undertone is not an exact science. There doesn't have to be a clear-cut answer. Undertone moves along a spectrum, with a whole range between. For example, on some websites and in some magazines, Lucy Liu, Angelina Jolie, Penelope Cruz, and Kerry Washington are labeled as warm, while on others, they're labeled as cool (which I believe is due more to lighting in photographs more than anything else).

So, what does undertone actually affect? Good question.

Undertone can effect everything from hair color to foundation color to lipstick tone to the clothes we subconsciously choose over others. It's why we leave some shirts shoved in the back of our dresser, and why some lipstick shades make us feel queasy when we see them upon our lips. Determining your undertone can be your greatest tool, and biggest weapon.

Undertone will help you to narrow down foundation shades much more quickly. Some lines skew toward a certain undertone (like Marc Jacobs and Dior, which tend to be more warm/peach), while other lines divide their products by undertone (like Make Up For Ever and MAC, just to name a few).

Undertone will also help you to choose the blush, shadow, lipstick, highlight, and contour shades that will be the most flattering to you skin.

The most important thing to keep in mind when it comes to undertone: do whatever makes you happy! Take your new knowledge, let it filter your world, but still wear that badass red lipstick that makes you feel vampy and confident and cool AF (even if it is cool and you're warm). Know the rules so you can break them.